Male spider mites keep a close watch on the juvenile females and tear off their outer skin as soon as they approach maturity so that they can be the first to mate with them.
Two-spotted spider mites (Tetranychus urticae) are a common agricultural pest that feeds on a wide range of crops, including beans and tomatoes. Mites shed their outer skin when they pass from juvenile nymphs to adults, a process known as molting.
Adult females can have multiple sexual partners, but only their first partner’s sperm fertilizes their eggs. As a result, there is strong competition between males for newly mature females.
To try to ensure this access, male mites often guard young females until they become fertile adults. This is a risky strategy because it costs energy and prevents males from foraging, and rival males can still steal females at the last minute.
Peter Schausberger at the University of Vienna in Austria and colleagues found that male guards try to minimize this risk by ripping off the outer skin of females just as they approach maturity so they can inseminate them before other males pounce on them.
The researchers filmed young spider mites that were reared individually in cages with or without the presence of a male.
Females that were housed without a male naturally shed their outer skin at a leisurely rate when they reached sexual maturity.
In contrast, females housed with a male had their outer skin forcibly removed. As the female approached maturity, the male began drumming on her skin to encourage her to open up. The male then used her mouthparts to rip off the female’s skin from behind her so he could expose her genitalia and immediately inseminate her.
This meant that females with a male emerged from their outer skin 5 minutes earlier on average than those alone, Schausberger says.
“Five minutes isn’t long in absolute time, but it is in relative time because these mites often live in high-density colonies where other males are nearby,” he says. “Every second is worth it when it comes to being first in the emerging woman.”
Spider mites are the first species in which this shedding behavior has been documented experimentally, says Schausberger.